DDS can be used in areas that require even the highest spectral purity. The Symmetricom Hydrogen Maser offers a DDS unit AOG-110 to generate a wide range of freq's from the 5MHz output. Their Phase Noise Measurement Test Sets is also based on a DDS design. a good DDS circuit can add as little as 3dB phase noise to a quality ref. The key is the reference. Of course by it's very nature any digitally signals will have some HF noise unless filtered. There are a number of cheap DDS boards on eBay but I have never played with any. I have a few freq standards with variable DSS output that can be real handy. Thomas Knox
> Date: Sun, 27 May 2012 18:08:41 -0400 > From: g...@partiallystapled.com > To: time-nuts@febo.com > Subject: [time-nuts] DDS in GPSDO design? > > Greetings, > > I've been pondering topologies for a custom GPSDO design and two obvious > choices seem to present themselves. The first, and seemingly more > popular by far, is to use a "pullable" oscillator as many OCXO and Rb > oscillators are and discipline it using a slow but precise DAC. But > unfortunately my Rb is not pullable so I would have to get another > oscillator. So I have a very stable but off-spec local oscillator, which > has to somehow be combined with the pulse-per-second from the GPS. If > there's a palatable analog way to do this, I'd love to hear, because it > would probably be simpler than the other idea. > > The second obvious idea is to use the local oscillator to clock a > frequency synthesizer (DDS). These can apparently tune a frequency very > finely and depending on how much one spends will produce a pretty clean > sine wave even at 10MHz. Since these also tend to require a FPGA it also > fits nicely with the nanosecond-level phase comparator I've been toying > with, and the whole mess (microcontroller, DDS, phase comp) can all be > clocked from some multiple of the LO without worrying about unwanted > phase correlation. Having the GPSDO be a black box that can transform > any undisciplined 10MHz reference into a disciplined one is very appealing. > > Does anyone have any comments or experience with DDS-based frequency > references? Are they too jittery for this type of application? It will > certainly require quite a lot of creative filtering -- one page I read > mentioned the pitfalls of tempco of phase shift -- but that's just a > good excuse to brush up on my analog design. > > -- m. tharp > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.